Colloidal gold solution and method for producing same

ABSTRACT

The purpose of the present invention is to provide a stable colloidal gold solution and a method for producing the stable colloidal gold solution. A colloidal gold solution which contains, in water, gold nanoparticles having particle diameters of 100 nm or less and anions represented by general formula (a); and a method for producing the colloidal gold solution. R—COO −  (a) (In the formula, R represents a linear or branched alkyl group having 1-4 carbon atoms.)

TECHNICAL FIELD

The present invention relates to a stable colloidal gold solution and a method for producing such a solution.

BACKGROUND ART

In recent years, gold nanoparticles have been used in many fields such as catalysts, pharmaceuticals, sensing, electronics, coloring materials, and painting materials. In these applications, a colloidal gold solution, which is a stable dispersion of gold nanoparticles in a solvent such as water, is often used as a raw material. In most cases, a colloidal gold solution is produced using chloroauric acid (HAuCl₄) as a raw material, and a large amount of chloride ions remain in the solution at the stage where chloroauric acid is reduced to form gold nanoparticles.

The use of gold nanoparticles to form a conductive paste or a catalyst has been studied. However, chloride ions remaining in a colloidal gold solution are not preferred because they can cause corrosion, catalyst poison, and other problems. Therefore, methods for desalination have been developed. However, for example, when desalination is performed using ion-exchange resin, a substantial amount of a gold colloid, which has a negative surface charge as chloride ions do, can also adsorb to ion-exchange resin. Therefore, the problem of a significant reduction in the concentration of gold nanoparticles in the colloid is pointed out (see, for example, Patent Document 1).

To avoid the problem caused by chloride ions as described above, there is proposed a method of producing a metal colloid from a metal salt free of halide ions such as chloride ions (see, for example, Patent Document 2). However, the method disclosed in Patent Document 2 and other general methods for preparing a precious metal colloid use a procedure in which a protective agent and a reducing agent are added after a metal salt or complex is dissolved in water. Therefore, such methods do not take into account the use of a metal salt or complex that cannot be completely dissolved in water.

There is also an example that includes adding gold acetate into a solution of alkyl diol, adding oleic acid and oleylamine thereto, and heating the mixture to 180° C. to form gold nanoparticles (see, for example, Non-Patent Document 1). Unfortunately, there has been no report yet on a colloid produced using gold acetate as a raw material and water as a solvent, because gold acetate is less soluble in water and recognized as being not usable as a raw material for aqueous colloids. Under these backgrounds, at present, very few materials other than chloroauric acid are available as raw materials for gold colloids.

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: Japanese Patent Laid-Open Publication No. 2009-120901

Patent Document 2: Japanese Patent Laid-Open Publication No. H11-151436

NON-PATENT DOCUMENT

Non-Patent Document 1: Monodispersed Core-Shell Fe3O4@Au Nanoparticles, L. Wan et al., J. Phys. Chem. B, 109 (2005) 21593-21601.

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

It is a principal object of the present invention to provide a stable colloidal gold solution containing water as a solvent. It is another principal object of the present invention to provide a method capable of simply preparing such a colloidal gold solution.

Means for Solving the Problems

As a result of earnest studies to solve the problems, the inventors have found that by adding ethanol to an aqueous dispersion of gold acetate, a red colloidal gold solution is obtained in a few minutes at room temperature. The present invention has been completed as a result of further studies based on this finding. Accordingly, the present invention provides colloidal gold solutions and methods for producing colloidal gold solutions, as described below.

-   Item 1. A colloidal gold solution, comprising: water; gold     nanoparticles with a particle size of 100 nm or less; and an anion     represented by general formula (a)

R—COO⁻  (a)

wherein R represents a linear or branched alkyl group of 1 to 4 carbon atoms.

-   Item 2. The colloidal gold solution according to Item 1, wherein the     anion is an acetate ion. -   Item 3. The colloidal gold solution according to Item 1 or 2,     further comprising a protective colloid. -   Item 4. The colloidal gold solution according to any one of Items 1     to 3, wherein the protective colloid is at least one selected from     the group consisting of polyvinylpyrrolidone, polyethylene glycol,     polyacrylic acid, poly(sodium acrylate), polyvinyl alcohol, and     carboxymethyl cellulose. -   Item 5. The colloidal gold solution according to any one of Items 1     to 4, further comprising a reducing agent comprising an alcohol with     a primary hydroxyl group and/or a secondary hydroxyl group. -   Item 6. The colloidal gold solution according to any one of Items 1     to 5, wherein the gold nanoparticles have a concentration of 0.0001     to 50% by weight. -   Item 7. The colloidal gold solution according to any one of Items 1     to 6, which is substantially free of chloride ions. -   Item 8. A method for producing a colloidal gold solution, comprising     the steps of: -   (i) dispersing a gold carboxylate in water to form a dispersion; and -   (ii) allowing a reducing agent to react with the gold carboxylate in     the dispersion obtained in the step (i) so that a colloidal gold     solution is obtained by reduction. -   Item 9. The method according to Item 8, wherein the reduction is     performed using at least one selected from the group consisting of     an alcohol with a primary hydroxyl group and/or a secondary hydroxyl     group, polyvinylpyrrolidone, polyvinyl alcohol, polyethylene glycol,     gelatin, starch, dextrin, carboxymethyl cellulose, methyl cellulose,     and ethyl cellulose. -   Item 10. The method according to Item 8 or 9, wherein the gold     carboxylate is gold acetate. -   Item 11. The method according to any one of Items 8 to 10, wherein     the dispersion contains a protective colloid. -   Item 12. The method according to any one of Items 8 to 11, wherein     the protective colloid is at least one selected from the group     consisting of polyvinylpyrrolidone, polyethylene glycol, polyacrylic     acid, poly(sodium acrylate), polyvinyl alcohol, and carboxymethyl     cellulose.

Advantages of the Invention

The present invention makes it possible to provide a colloidal gold solution that is stable over a long period without causing precipitation and the like. More specifically, there is provided a colloidal gold solution that is produced using a gold carboxylate as a source of gold nanoparticles, substantially free of chloride ions, and such that the anion of formula (a) above adsorbs onto the surface of gold nanoparticles to enable them to be stably dispersed in water.

The method of the present invention for producing a colloidal gold solution also makes it possible to obtain a stable, chloride ion-free, colloidal gold solution. Therefore, this eliminates the need for a post-treatment for removing chloride ions and makes it possible to simply obtain a high-concentration, colloidal gold solution with high yield. According to the method of the present invention, coarse particles are not produced as by-products of the process.

Under conventional conditions for preparing a metal colloid, a solution containing a completely dissolved metal salt is usually used as a starting material. Very few examples have been reported for the use of a hardly-soluble metal salt in the preparation of a metal colloid. Particularly concerning a colloidal gold solution, there has been no report yet of the production of a high-concentration, colloidal gold solution using a hardly-soluble metal salt as a raw material. However, the present invention makes is possible to prepare a high-concentration, colloidal gold solution using a simple system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the UV-VIS spectrum of a gold colloid prepared with gold acetate in Example 1.

FIG. 2 is a TEM photograph of a gold colloid prepared by using gold acetate and removing PVP in Example 1 and a graph showing the size distribution of gold particles in the gold colloid.

FIG. 3 is a graph showing the UV-VIS spectrum of a gold colloid prepared with gold acetate in Example 2.

FIG. 4 is a TEM photograph of a gold colloid prepared by using gold acetate and removing PVP in Example 2 and a graph showing the size distribution of gold particles in the gold colloid.

EMBODIMENTS THE INVENTION 1. Colloidal Gold Solution

The colloidal gold solution of the present invention contains gold nanoparticles with a particle size of 100 nm or less and an anion represented by general formula (a) below in water.

R—COO⁻  (a),

wherein R represents a linear or branched alkyl group of 1 to 4 carbon atoms.

In the description, an anion represented by general formula (a) above is also referred to as a “carboxylate,” and a gold salt thereof is also referred to as a “gold carboxylate.”

If a colloidal gold solution contains halide ions such as chloride ions, for example, the use of the solution as a conductive paste or a catalyst can cause a problem such as corrosion or catalyst poison. Therefore, a halide ion-free gold compound should preferably be used as a source of gold nanoparticles. In the present invention, the material used as a source of gold nanoparticles is preferably carboxylated gold (namely, a gold carboxylate), more preferably carboxylated trivalent gold. Examples of such a gold compound include Au(CH₃COO)₃, Au(C₂H₅COO)₃, and Au(HCOO)₃. The gold carboxylate may include a basic salt such as Au(OH)(CH₃COO)₂ or Au(OH)₂(CH₃COO). These examples of the gold carboxylate may be used singly or in combination of two or more. Among these examples of the gold carboxylate, gold acetate (Au(CH₃COO)₃) is preferred because it is easily available and has a suitable level of water solubility.

The gold nanoparticles contained in the colloidal gold solution of the present invention have an average particle size of 100 nm or less, preferably 50 nm or less, more preferably 2 to 40 nm. As used herein, the term “average particle size” refers to the value obtained by calculating the number average from a size distribution observed with a transmission electron microscope (TEM).

The colloidal gold solution of the present invention generally has a gold nanoparticle concentration of 0.0001 to 50% by weight, preferably 0.001 to 10% by weight, more preferably 0.01 to 5% by weight. Among the total amount of gold used to form the colloidal gold solution, the concentration of the gold nanoparticles having plasmon absorption in the solution can be determined as follows. Several known samples with different gold nanoparticle concentrations are prepared with the same medium (when PVP or the like is added to water, the concentration of PVP or the like should be the same) and then subjected to UV-VIS measurement. The measured optical density (OD) values at the wavelength (α_(max)) where the maximum plasmon absorption occurs are plotted against the concentrations. The resulting calibration curve (which can be approximated by a straight line) is used to determine the concentration of the gold nanoparticles.

In the colloidal gold solution of the present invention, the anion of general formula (a) below may be dissolved in the colloidal gold solution or adsorbed on the surfaces of the gold nanoparticles.

R—COO⁻  (a)

In the formula, R represents hydrogen or a linear or branched alkyl group of 1 to 4 carbon atoms, preferably 1 to 2 carbon atoms, more preferably one carbon atom. Specifically, the alkyl group may be methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, tert-butyl, or the like, preferably methyl. The anion of general formula (a) above is preferably an acetate ion (CH₃COO⁻).

The colloidal gold solution of the present invention may also contain a protective colloid. The protective colloid may be appropriately selected from those conventionally known in the art. Examples of the protective colloid include colloids of polyvinylpyrrolidone (PVP), polyvinyl alcohol, polyethylene glycol, polyacrylic acid, poly(sodium acrylate), gelatin, starch, dextrin, carboxymethyl cellulose, methyl cellulose, ethyl cellulose, and glutathione. In particular, polyvinylpyrrolidone, polyethylene glycol, polyacrylic acid, poly(sodium acrylate), polyvinyl alcohol, and carboxymethyl cellulose are preferred, and polyvinylpyrrolidone and polyethylene glycol are more preferred. The colloidal gold solution may contain one of these protective colloids alone or two or more of these protective colloids. As long as the present invention remains effective, these protective colloids may be denatured or modified or undergo other changes. When a polymer is used to form the protective colloid, the polymer may have any molecular weight as long as the present invention remains effective. For example, when polyvinylpyrrolidone is used, it may be specifically PVP K-15 (10,000 in average molecular weight), K-30 (40,000 in average molecular weight), or K-90 (360,000 in average molecular weight) manufactured by Kishida Chemical Co., Ltd.

When the colloidal gold solution of the present invention contains the protective colloid, the content of the protective colloid may be at any level equal to or lower than its solubility in water. The content of the protective colloid is generally from 0.1 to 1,000 moles, preferably from 0.1 to 500 moles, more preferably from 0.1 to 100 moles, based on 1 mole of gold. When the protective colloid is a polymer, the molar content of its monomer unit should be taken into account.

In the colloidal gold solution of the present invention, water is used as a solvent. In the solution, water may be of any type. Water free of impurities such as chloride ions is preferably used, such as distilled water, ion-exchanged water, purified water, pure water, or ultrapure water.

The colloidal gold solution of the present invention may also contain a reducing agent. The reducing agent may be appropriately selected from those conventionally known in the art. Examples of the reducing agent include alcohols having a primary hydroxyl group, such as methanol, ethanol, 1-propanol, and ethylene glycol; alcohols having a secondary hydroxyl group, such as 2-propanol and 2-butanol; alcohols having both primary and secondary hydroxyl groups, such as glycerin; aldehydes such as formaldehyde and acetaldehyde; sugars such as glucose, fructose, glyceraldehyde, lactose, arabinose, and maltose; organic acids and salts thereof, such as citric acid, sodium citrate, potassium citrate, ammonium citrate, tannic acid, ascorbic acid, sodium ascorbate, and potassium ascorbate; boron hydride and salts thereof such as sodium borohydride and potassium borohydride; and hydrazine and inorganic salts thereof, such as hydrazine, hydrazine hydrochloride, and hydrazine sulfate. These reducing agents may be used singly or in combination of two or more. Among these reducing agents, alcohols having a primary hydroxyl group and/or a secondary hydroxyl group are preferred, and ethanol and methanol, and the like are more preferred. Some types of protective colloids may also be used as reducing agents. Examples of protective colloids that may also be used as reducing agents will be shown below in the section “Step (i).”

When the colloidal gold solution of the present invention contains a reducing agent, the content of the reducing agent may be at any level as long as the present invention remains effective. Based on 1 mole of gold, for example, the content of the reducing agent may be from 1 to 100,000 moles, preferably from 1 to 50,000 moles, more preferably from 1 to 20,000 moles.

Preferably, the colloidal gold solution of the present invention is substantially free of halide ions (X) selected from fluoride, chloride, bromide, and iodide ions. In this case, problems such as halide ion-induced corrosion and the action of halide ions as a catalyst poison can be prevented even when the colloidal gold solution of the present invention is used as a conductive paste, a catalyst, or the like. As used herein, the term “substantially free of halide ions” means that the amount of halide ions is smaller than the amount of gold in the colloidal gold solution. For example, when chloroauric acid (HAuCl₄) is used in the preparation of a colloidal gold solution, the resulting molar ratio (X/Au) of halide ions (X) to gold (Au) will be usually about 4 unless a treatment for removing chloride ions is performed. In the present invention, however, the molar ratio (X/Au) of halide ions (X) to gold (Au) in the colloidal gold solution can be set at, for example, 0.4 or less, 0.04 or less, or 0.004. When a halide ion-containing raw material such as chloroauric acid is used, the produced colloidal gold solution can be subjected to desalination treatment or the like so that the halide ion concentration can be reduced. However, the halide ion concentration of the colloidal gold solution of the present invention can be set at a predetermined level or less without any desalination treatment.

The colloidal gold solution of the present invention may also contain a conventionally known additive depending on the intended use. Examples of such an additive include a colorant, a stabilizer, a surfactant, a dispersing agent, a thicker, and the like.

The colloidal gold solution of the present invention is suitable for use in applications such as conductive ink, conductive paste, catalysts, sensors, joining materials, coloring materials, painting materials, and biomarkers.

2. Method for Producing Colloidal Gold Solution

The present invention provides a method for producing a colloidal gold solution, which makes it possible to simply prepare the colloidal gold solution described above. The method for producing a colloidal gold solution includes the steps (i) and (ii) described below.

Step (i)

The step (i) includes dispersing a gold carboxylate in water to form a dispersion. In this step, the gold carboxylate and water as a solvent are as described above in the section “1. Colloidal gold solution.”

In the step (i), a solvent comprising water and a protective colloid may be used to disperse the gold carboxylate. When the protective colloid is added, gold nanoparticles with a narrow particle size distribution can be produced, and a highly stable colloidal gold solution can be obtained. Specifically, the protective colloid is as described above in the section “1. Colloidal gold solution.”

As mentioned above, for example, a protective colloid of polyvinylpyrrolidone, polyvinyl alcohol, polyethylene glycol, gelatin, starch, dextrin, carboxymethyl cellulose, methyl cellulose, ethyl cellulose, or the like can also be used as a reducing agent. In particular preferred are polyvinylpyrrolidone, polyethylene glycol, polyacrylic acid, poly(sodium acrylate), polyvinyl alcohol, and carboxymethyl cellulose. Polyvinylpyrrolidone and polyvinyl alcohol are more preferred because they can form a more stable colloidal gold solution. In the step (i), when any of these materials may be previously added as a protective colloid to the dispersion, a colloidal gold solution can be prepared in the step (ii) without adding any additional reducing agent.

The content of the protective colloid may be at any level equal to or lower than its solubility in water. The content of the protective colloid is generally from 0.1 to 1,000 moles, preferably from 0.1 to 500 moles, more preferably from 0.1 to 100 moles, based on 1 mole of gold.

In the step (i), the gold carboxylate may be dispersed in water by any suitable method selected from methods commonly used to disperse a powder in water. For example, a magnetic stirrer, a touch mixer, an ultrasonic cleaner, or the like may be used. These devices may also be used in any combination. Examples of the dispersing conditions include, but are not limited to, dispersing treatment with a touch mixer (240 rpm, 10 seconds) followed by dispersing treatment with an ultrasonic cleaner (60 seconds). Such a process may be repeated one or more times (typically 1 to 20 times, preferably 5 to 10 times).

For example, when gold acetate is used as the gold carboxylate, the resulting dispersion is brown and exhibits a Tyndall effect, by which colloidal dispersion can be confirmed.

The dispersion obtained in the step (i) may have any concentration as long as it has the dispersed gold carboxylate concentration required to form the desired colloidal solution. In view of the stability of the produced colloidal gold solution, the concentration is generally such that the dispersion contains 0.0001 to 50% by weight of gold metal, preferably such that the dispersion contains 0.001 to 10% by weight of gold metal.

A gold carboxylate is known to be slightly soluble as ions in water. For example, the solubility of gold acetate is reported to be 10⁻⁵ M or less in Non-Patent Document 2 (G. C. Bond et al., Chapter 4, Preparation of Supported Gold Catalysts, Catalysis by Gold (Series editor, G. J. Hutchings), p. 89, Imperial College Press (2006)). With no intention to limit the interpretation of the present invention, it is conceivable that if a gold carboxylate is dispersed in water, it may be partly dissolved in water with the remainder dispersed as a colloid in water.

Step (ii)

The step (ii) includes allowing a reducing agent to react with the gold carboxylate in the dispersion obtained in the step (i) so that a colloidal gold solution is obtained by reduction.

In the step (ii), the reduction of the gold carboxylate can be performed by adding a reducing agent to the dispersion obtained in the step (i). The reducing agent is as described above in the section “1. Colloidal gold solution,” preferably an alcohol with a primary hydroxyl group and/or a secondary hydroxyl group, more preferably ethanol, methanol, or the like. The amount of the reducing agent allowed to react with the dispersion is typically from 1 to 100,000 moles, preferably from 1 to 50,000 moles, more preferably from 1 to 20,000 moles based on 1 mole of gold, although it is not restricted as long as the ratio of the number of moles of the reducing agent to the number of moles of gold in the dispersion is at least the oxidation-reduction equivalent. For example, when the reducing agent is an alcohol, its volume ratio is preferably in the range where it can be uniformly dissolved in water, and such a volume ratio can be appropriately selected based on the number of moles.

The reducing agent may be allowed to react with the dispersion under any conditions where the gold carboxylate and the reducing agent can react in the dispersion. If necessary, stirring and other processes may be performed. The reaction temperature is also not restricted and may be appropriately selected depending on the type of the reducing agent to be used and other conditions. The reaction temperature is typically from 1 to 100° C., preferably from 5 to 40° C., more preferably from 10 to 30° C.

For example, when a relatively strong reducing agent such as ethanol or methanol is used, the reaction can sufficiently proceed under room temperature conditions in a few minutes. However, when a weak reducing agent such as 2-propanol or polyvinylpyrrolidone, which can also serve as a protective colloid, is used, it may take a few days to a few months to perform the reaction under room temperature conditions. Therefore, when a weak reducing agent is used, the reaction solution may be heated so that a gold colloid can be produced in a few minutes to a few hours by increasing the reaction temperature and the reaction rate. The heating temperature is not restricted as long as a gold colloid can be produced. The heating temperature is typically from 40 to 100° C., preferably from 60 to 100° C., more preferably from 80 to 100° C. More specifically, when gold acetate is used as the gold carboxylate in combination with polyvinylpyrrolidone as the protective colloid and the reducing agent, the mixture may be heated at 80 to 100° C. for 5 to 60 minutes so that a high-concentration colloidal gold solution can be obtained.

In the step (ii), if necessary, excess of the protective colloid may be removed. The removal method is not restricted and may be appropriately selected from conventionally known methods. For example, the removal can be performed using centrifugal filtration, membrane separation, dialysis, electrodialysis, or the like. Any of these processes may be performed twice or more depending on the amount of the protective colloid to be removed. If necessary, the resulting colloidal gold solution may also be subjected to pH adjustment, concentration, purification, or other processes according to conventionally known methods.

As mentioned above, the method of the present invention for producing a colloidal gold solution may include dispersing the gold carboxylate in water containing the protective colloid as needed and then allowing the reducing agent to react. More simply, the protective colloid and the reducing agent may be added to water as a solvent to form an aqueous solution, and the gold carboxylate may be added thereto to form a colloidal gold solution. Even when such a method is used, the same materials and conditions as those described above may be used.

In the method of producing a colloidal gold solution of the present invention, a preferred combination of materials includes, for example, gold acetate as the gold carboxylate, polyvinylpyrrolidone and/or polyvinyl alcohol as the protective colloid, and methanol and/or ethanol as the reducing agent. The use of such a combination makes it possible to simply form a stable, high-concentration, colloidal gold solution.

With no intention to limit the interpretation of the present invention, a reason why a high-concentration gold colloid can be obtained according to the present invention may be, for example, as follows. A gold carboxylate such as gold acetate is hardly soluble in water. Therefore, gold ions in water would be kept at a low concentration suitable for the production of a gold colloid. When the reducing agent acts to reduce gold ions to gold metal, a small amount of the gold carboxylate would become soluble in water again. Therefore, a small amount of the gold carboxylate would be repeatedly dissolved and reduced to form a gold colloid gradually, and as a result, it is conceivable that a high-concentration gold colloid can be obtained.

EXAMPLES

Hereinafter, the present invention will be more specifically described with reference to examples and comparative example, which however are not intended to limit the present invention.

Example 1

Fifty mg of polyvinylpyrrolidone (PVP K15 manufactured by Kishida Chemical Co., Ltd.) was dissolved in 10 mL of an ethanol/water (1:1) solution. Five mg of brown powder of gold acetate (Au(CH₃COO)₃ manufactured by Alfa Aesar, 99.99% purity (shown in the manufacturer's certificate of analysis)) was added to the solution and then dispersed using a touch mixer (Vortex Genius 3 manufactured by IKA) and an ultrasonic cleaner (US-2R manufactured by AS ONE Corporation). Standard conditions for the dispersing treatment were the touch mixer treatment (at 2,400 rpm for 10 seconds) and the ultrasonic cleaner treatment (for 60 seconds) alternately performed each 5 to 7 times. However, the number of cycles was increased or decreased as needed depending on the degree of precipitation of the residue. After these treatments, the undissolved precipitate substantially disappeared from the solution, but a Tyndall effect was observed when light was applied to the side of the vessel from an LED light. This showed that the product was not a true aqueous solution but a brown colloidal dispersion.

The dispersion was allowed to stand at room temperature (about 24° C.) so that the liquid started to turn red in about 10 minutes. After standing overnight, a red colloidal gold solution (prepared concentration: 1.3 mmol-Au/L) was obtained. The colloidal solution was diluted with water to 1/10 concentration. The diluted solution was placed in a quartz cell with an optical path length of 1 cm and subjected to UV-VIS spectrometry with a spectrophotometer (UV-1800 manufactured by Shimadzu Corporation). FIG. 1 shows the results.

In FIG. 1, the horizontal axis is the wavelength and the vertical axis is the value indicated as absorbance by the spectrophotometer. The absorbance of a colloidal gold solution includes not only absorption but also the effect of scattering and reflection. Therefore, the term “optical density” is used instead herein. The measured spectrum clearly showed a surface plasmon absorption peak at a wavelength (λ_(max)) of 527 nm.

It is known that there is such a relationship that the peak wavelength of plasmon absorption generally decreases with decreasing gold nanoparticle size. For example, Non-Patent Document 3 (KOBAYASHI Toshikatsu, Chapter 21, “Concentrated Gold Nanoparticle Paste as Colorant,” Gold Nanotechnology (supervised by HARUTA Masatake), pp. 273-282, CMC Publishing Co., Ltd. (2009)) shows the volume average particle size (D_(Au)) of gold nanoparticles and the corresponding plasmon absorption wavelength (λ_(max)). If the same dispersion medium is used for gold colloids, there is a linear relationship between D_(Au) and λ_(max) values, and in principle, D_(Au) can be determined from measured λ_(max).

In the examples, water is used as the dispersion medium for the gold colloid. However, if a reducing agent such as ethanol or a protective colloid such as PVP is added to the dispersion medium, the dielectric constant of the dispersion medium will change, so that λ_(max) will change even with the same D_(Au). Additionally, if D_(Au) is in the range of 50 nm or less, the plasmon absorption will increase with increasing D_(Au). Therefore, if the particle size distribution is wide, the relationship between D_(Au) and λ_(max) will be more strongly influenced by lager particles and thus differ from that in the monodisperse case. As in the examples, therefore, if the dispersion medium composition differs case by case and the gold nanoparticle size distribution is not monodisperse, it will be difficult to correctly determine D_(Au) from λ_(max).

Therefore, the relationship between the D_(Au) value and λ_(max), determined from the TEM measurement in Examples 1 and 2 below, were added to the series of data points shown in FIG. 8 of Non-Patent Document 3, and the least-squares method was used to calculate a regression line. The resulting correlation coefficient was 0.96 (considerably close to 1), which indicates that the state of the gold colloid according to the present invention is similar to the state of the gold colloid shown in FIG. 8 of Non-Patent Document 3. Using the resulting linear expression, the D_(Au) range corresponding to the λ_(max) range (524-562 nm) measured in Example 1 and Examples 2 to 33 shown below was determined to be 12-37 nm. It was estimated that colloidal gold with an average particle size of about 10 to about 40 nm are obtained in the examples.

In addition, the optical density at λ_(max) can be used as a measure of gold colloid concentration. The maximum equivalent optical density (or OD_(max)) is calculated from formula (1) below, wherein L (cm) is the optical path length of the quartz cell used in the measurement, F is the degree of dilution of the solution (for example, when the concentration is diluted to 1/10 in the measurement, F=10), and Y is the optical density value at λ_(max). Immediately after the preparation, the OD_(max) calculated from formula (1) was 6.8 (see Table 1 below).

OD_(max) =Y×F/L   formula (1)

The colloidal gold dispersion (40 days after the preparation) was subjected to centrifugal filtration for removal of PVP. The dispersion was centrifugally treated at 4,000 rpm for 20 minutes using a centrifugal filter unit (Amicon Ultra-15 manufactured by Millipore Corporation) with a cut-off molecular weight of 50,000. As a result, the fraction of the colloidal gold that did not pass through the filter was concentrated to a dark red dispersion, and the filtrate passing through the filter was colorless and clear. The concentrated dispersion was diluted to 10 mL with water, and the dilution was subjected centrifugal filtration again. This process was repeated three times. To check whether or not PVP remained in the filtrate, an Evans blue (EB) solution was dropped into the filtrate and then the UVVIS absorption was measured according to the description in Non-Patent Document 4 (Binding of Evans Blue Onto Poly(N-Vinyl-2-Pyrrolidone), M. Maruthamuthu and E. Subrarnanian, Polymer Bulletin 14, 207-212 (1985)).

The first filtrate showed absorption at 639 nm produced by the PVP-EB complex. However, the third filtrate showed no absorption at 639 nm, and only showed absorption at 609 nm produced by free EB. Therefore, it was demonstrated that the three times of centrifugal filtration successfully removed PVP. Evans blue was also added to part of the resulting colloidal gold dispersion, and then the UV-VIS absorption was measured. As a result, no absorption was observed at 639 nm, which showed that no PVP remained in water serving as the dispersion medium for the gold colloid. After the removal of PVP, λ_(max) was 529 nm in the UVVIS absorption spectrum of the colloidal gold solution (Evans bluefree aliquot).

After the removal of PVP, the colloidal gold solution was dropped onto a microgrid, dried, and observed with a transmission electron microscope (TEM). FIG. 2 shows the TEM photograph. A mixture of gold nanoparticles with different crystal structures (icosahedral structure Ih, pentagonal decahedron structure Dh, face-centered cubic lattice structure Fcc) was observed, in which the particle size ranged from about 5 nm to more than 20 nm. The number average particle size determined from the size distribution in FIG. 2 was 11.5 nm.

Subsequently, the colloidal gold solution was stored at room temperature. Even after 7 months, the colloidal solution was stable with no precipitation observed. This suggested that gold nanoparticles and acetate ions and/or a small amount of the PVP residue adsorbed on the surface of the gold nanoparticles form a stable colloidal gold solution. Table 1 below shows a summary of the λ_(max) and OD_(max) values immediately after the preparation, after the removal of PVP, and after 7 months.

TABLE 1 λ_(max) (nm) OD_(max) Immediately after preparation 527 6.8 After removal of PVP 529 8.6 After 7 months 529 8.2

Table 1 shows that the particle size and the concentration fluctuated at the time of removal of PVP, but after that, the colloid was stable with no significant change over 7 months.

Comparative Example 1

A solution was prepared as in Example 1, except that 0.13 mL of an aqueous solution of 0.1 mol/L chloroauric acid (HAuCl₄) was used, which was prepared by weighing, with an electronic balance, chloroauric acid tetrahydrate (Kishida Chemical Co., Ltd.) crystals instead of gold acetate as a raw material (gold carboxylate) for a gold colloid and then dissolving the crystals in a given amount of water. The as-prepared liquid was yellow (the normal color of an aqueous chloroauric acid solution) and showed no Tyndall effect. The liquid was a true solution in which chloroauric acid was completely dissolved. As in Example 1, the solution was allowed to stand at room temperature overnight. However, no change in color occurred, and the solution remained yellow with no production of a gold colloid.

Example 2

Six g of polyvinylpyrrolidone (PVP K15) was dissolved in 20 mL of water, and 2 mL of the solution was placed in a glass screw vial. To the vial was added 20 mg of gold acetate powder. The mixture was dispersed using a touch mixer and an ultrasonic cleaner under the same conditions as in Example 1. As a result, a brown dispersion was obtained. A stirring bar was placed in the vial, and the vial was sealed with a cap with a Teflon-coated packing. The solution was boiled with stirring on a hot plate stirrer. As a result, a red colloidal liquid (Au 26.7 mmol/L) was obtained in a few minutes. FIG. 3 shows the UVVIS spectrum of the 1/100 diluted colloidal liquid (absorption peak at 530 nm). The gold particle size calculated from formula (1) was 18.1 nm.

After a lapse of 40 days from the preparation, 2 mL of the colloidal liquid was added to 10 mL of water to form 12 mL of a dilution. Under the same conditions as in Example 1, the dilution was subjected to centrifugal filtration six times so that PVP was removed. After the sixth filtration, the amount of added water was controlled so that a colloidal gold solution with a concentration substantially the same as at the time of the preparation was obtained. After the removal of PVP, λ_(max) was 524 nm in the UV-VIS absorption of the colloidal gold solution (Evans blue-free aliquot). Table 2 below shows the λ_(max) and OD_(max) values obtained by the same method as in Example 1 immediately after the preparation, after the removal of PVP, and after 7 months.

TABLE 2 λ_(max) (nm) OD_(max) Immediately after preparation 530 60 After removal of PVP 524 75 After 7 months 524 75

Table 2 shows that as in Example 1, the particle size and the concentration fluctuated at the time of removal of PVP, but after that, the colloid was stable with no significant change over 7 months.

FIG. 4 shows a TEM photograph of the gold colloid after the removal of PVP. FIG. 4 shows that as in Example 1, a mixture of gold nanoparticles with different structures was also obtained in Example 2. The resulting particle size distribution was slightly better than that in Example 1. The average particle size determined from the size distribution in FIG. 4 (the arithmetic mean usually determined with TEM) was 9.8 nm.

Comparative Example 2

A solution was prepared as in Example 2, except that 0.5 mL of an aqueous solution of 0.1 mol/L chloroauric acid (HAuCl₄), which was prepared with chloroauric acid tetrahydrate (Kishida Chemical Co., Ltd.) by the same method as in Comparative Example 1, was used instead of using 20 mg of gold acetate powder. When the solution was boiled under reflux, coarse gold particles precipitated, and no gold colloid formed.

Example 3 to 6

In a test tube, 25 mg of polyvinylpyrrolidone (PVP K15) was dissolved in 2.5 mL of water. Five mg of gold acetate powder was added to the solution. The mixture was dispersed using a touch mixer (Vortex Genius 3 manufactured by IKA) and an ultrasonic cleaner (AU-25C manufactured by Aiwa Ika Kogyo K.K.) under the same conditions as in Example 1. To the dispersion was added 2.5 mL of one of the reducing agents shown in Table 3 (ethanol for Example 3, methanol for Example 4, ethylene glycol for Example 5, 2-propanol for Example 6) and stirred. After the tube was capped, the dispersion was allowed to stand at room temperature. As a result, a red gold colloid formed in Examples 3 to 5. In Example 6, no change was observed even after standing at room temperature for one day, but a gold colloid quickly formed when the capped tube was heated to 100° C. in boiling water. After the formation, the colloid was allowed to stand at room temperature for 2 days and then subjected to UVVIS measurement. Using the measured λ_(max) value, the OD_(max) value was calculated from formula (1) above. Table 3 below shows these values.

Comparative Example 3

The same procedure was performed as in Examples 3 to 6, except that 2.5 mL of tert-butanol was used as the reducing agent. At room temperature, no change was observed ever after standing for 1 day. The product was further heated for 2 hours in boiling water, but a brown precipitate formed with a clear supernatant, and no gold colloid formed. Table 3 below also shows the results.

TABLE 3 Temperature λ_(max) Reducing agent (° C.) (nm) OD_(max) Example 3 Ethanol 24 528 8.6 Example 4 Methanol 24 532 6.1 Example 5 Ethylene glycol 24 528 4.9 Example 6 2-propanol 100 526 4.2 Comparative Tert-butanol 100 None — Example 3

Table 3 shows that in all cases where an alcohol with a primary or secondary hydroxyl group was used as a reducing agent (Examples 3 to 6), gold acetate was successfully reduced to form a gold colloid, but no reduction occurred when tert-butanol only with a tertiary hydroxyl group was used.

Example 7 to 16 Amounts of Gold Acetate and Dispersing Agent

In a test tube, the specified amount of polyvinylpyrrolidone (PVP K15) shown in Table 4 was dissolved in 2.5 mL of water. The specified amount of gold acetate powder shown in Table 4 was added to the resulting solution. The mixture was dispersed using a touch mixer (Vortex Genius 3 manufactured by IKA) and an ultrasonic cleaner (AU-25C manufactured by Aiwa Ika Kogyo K.K.) under the same conditions as in Example 1. To the dispersion was added 2.5 mL of ethanol. After the test tube was capped and allowed to stand at room temperature, a gold colloid formed. After standing for 1 day, the UV-VIS spectrum was measured. Table 4 below shows the λ_(max) and OD_(max) values obtained by the same method as in Example 1. In table 4 below, the Au concentration is the weight concentration of Au in the colloidal solution, which is calculated from the amount of gold acetate used in the preparation. PVP/Au is the molar ratio calculated based on the amount of the monomer unit (111 in molecular weight) of PVP.

TABLE 4 Gold PVP Au PVP/Au acetate K15 concentration (molar λ_(max) (mg) (mg) (wt %) ratio) (nm) OD_(max) Example 7 5.3 0 0.06 0.0 526 5.4 Example 8 5.3 1.1 0.06 0.7 526 8.3 Example 9 5.3 5.2 0.06 3.3 526 13.7 Example 10 5.3 25.3 0.06 16 528 9.6 Example 11 5.3 100 0.06 64 530 7.4 Example 12 5.3 500 0.06 318 536 9.0 Example 13 1.2 5.4 0.01 15 528 1.5 Example 14 5 24.9 0.06 17 528 8.7 Example 15 50 250 0.55 17 530 66.0 Example 16 500 2503 3.5 17 534 193.9

In some conventional examples of precious metal colloid preparation, the precious metal particle size is significantly changed by the addition amount of PVP (see, for example, Japanese Patent Laid-Open Publication No. 2005-281817). It is suggested, however, that even if the amount of PVP added to gold acetate is significantly changed, λ_(max) will not significantly change, and therefore, the gold nanoparticle size will not significantly change, either. Although a gold colloid formed regardless of addition of PVP, the OD_(max) value obtained in Examples 8 to 12 with the addition of PVP was at least 1.4 times higher than that obtained in Example 7 without the addition of PVP even when the same amount of gold acetate was used. In Examples 13 to 16, the amount of gold acetate was changed. The resulting OD_(max) value had a substantially linear relationship with the concentration of gold nanoparticles until the concentration of gold nanoparticles reached 0.55% by weight. In Example 16, the resulting value was about 50% of the OD_(max) value expected by extrapolation of the straight line.

Example 17 to 24 Effect of Protective Colloid During Reduction by Ethanol

In a test tube, 25 mg of one of different dispersing agents shown in Table 5 was dissolved in 2.5 mL of water. Five mg of gold acetate powder was added to the solution. The mixture was dispersed using a touch mixer (Vortex Genius 3 manufactured by IKA) and an ultrasonic cleaner (AU-25C manufactured by Aiwa Ika Kogyo K.K.) under the same conditions as in Example 1. To the dispersion was added 2.5 mL of ethanol. After the test tube was capped and allowed to stand at room temperature, a gold colloid formed. After standing for 1 day, the UV-VIS spectrum was measured. Using the measured λ_(max) value, the OD_(max) value was calculated from formula (1) above. Table 5 shows these values.

TABLE 5 λ_(max) Protective colloid (nm) OD_(max) Example 17 PVP K-15 536 5.44 (manufactured by Kishida Chemical Co., Ltd, 10,000 in average molecular weight) Example 18 PVP K-30 526 5.64 (manufactured by Kishida Chemical Co., Ltd, 40,000 in average molecular weight) Example 19 PVP K-90 526 4.22 (manufactured by Kishida Chemical Co., Ltd, 360,000 in average molecular weight) Example 20 PEG20M 536 3.42 (polyethylene glycol 20M) (manufactured by Kishida Chemical Co., Ltd., 18,000-25,000 in average molecular weight) Example 21 PAA-Na 562 2.52 (poly(sodium acrylate)) (manufactured by Wako Pure Chemical Industries, Ltd., 2,700-7,500 in polymerization degree) Example 22 PAA (polyacrylic acid) 556 1.03 (manufactured by Wako Pure Chemical Industries, Ltd., about 5,000 in average molecular weight) Example 23 PVA (polyvinyl alcohol) 536 3.23 (PV-217 available from AS ONE Corporation) Example 24 CMC (carboxymethyl cellulose) 536 4.38 (CMF-7 available from AS ONE Corporation)

Table 5 shows that the λ_(max) of the resulting gold colloid varied with the type of protective colloid. More specifically, it was shown that when PVP K-30 (Example 18) or PVP K-90 (Example 19) with a molecular weight higher than that of PVP K-15 (Example 17) was used, lower λ_(max) and comparatively smaller particle size gold colloids were obtained. It was also shown that when poly(sodium acrylate) or polyacrylic acid was used (Example 21 or 22), higher λ_(max) and larger particle size gold colloids were obtained. It was further shown that when polyethylene glycol, polyvinyl alcohol, or carboxymethyl cellulose was used (Example 20, 23, or 24), the resulting λ_(max) and the resulting colloidal gold particle size were substantially the same as those obtained with PVP K-15 (Example 17), but the resulting OD_(max) value was lower than that obtained with PVP K-15, which indicated a slight reduction in the gold nanoparticle concentration.

Example 25 to 33 Reduction by Different Protective Colloids

In a test tube, the specified amount of the dispersing agent shown in Table 6 was added to 5 mL or 2 mL of water and dissolved. The specified amount of gold acetate powder was added to the solution. The mixture was dispersed using a touch mixer (Vortex Genius 3 manufactured by IKA) and an ultrasonic cleaner (AU-25C manufactured by Aiwa Ika Kogyo K.K.) under the same conditions as in Example 1. A gold colloid formed while the capped tube was heated for 2 hours in boiling water. Subsequently, the colloid was allowed to stand at room temperature for 1 day (or 3 days in Examples 30 to 33, or allowed to stand at room temperature for 5 days without heating in Example 26), and then the UVVIS spectrum was measured. Using the measured λ_(max) value, the OD_(max) value was calculated from formula (1) above. Table 6 below shows these values.

TABLE 6 Au Gold Protective concen- Water acetate colloid tration λ_(max) (mL) (mg) Type (mg) (wt %) Heating (nm) OD_(max) Example 5 1 PVP 5 0.01 Heating 524 0.8 25 K-1.5 100° C. Example 5 4.7 PVP 25.5 0.05 No 534 3.2 26 K-1.5 heating Example 5 4.9 PVP 25.1 0.05 Heating 530 6.7 27 K-1.5 100° C. Example 5 50.1 PVP 251 0.50 Heating 534 81.6 28 K-1.5 100° C. Example 5 500 PVP 2500 3.3 Heating 534 116.6 29 K-1.5 100° C. Example 2 20 PVP 600 0.40 Heating 556 69.6 30 K-30 100° C. Example 2 20 PEG 600 0.40 Heating 552 12.4 31 20M 100° C. Example 2 20 PVA 600 0.40 Heating 534 108 32 100° C. Example 2 20 CMC 600 0.40 Heating 550 14.6 33 100° C. * In the table, PEG, PVA, and CMC are all the same as those shown in Table 5 above.

Table 6 shows that even when ethanol was not added, a gold colloid was successfully produced using PVP, PEG, PVA, or CMC as a protective colloid. A comparison between Examples 26 and 27 shows that the colloid production at room temperature required a long time and resulted in a low OD_(max) value, whereas heating made it possible to obtain a colloid with a higher OD_(max) value in a shorter time. In addition, an increase in the amount of gold acetate made it possible to produce a colloid with an OD_(max) value of more than 100 (Examples 29 and 32). When PVP or PVA was used as a protective colloid, the resulting gold colloid was stable even after standing for 3 days (Examples 25 to 30 and 32).

CONCLUSION

The above results show that the present invention makes it possible to obtain a stable colloidal gold solution. The present invention also makes it possible to prepare a colloidal gold solution with a concentration significantly higher than that of conventional colloidal gold solutions. It has also been demonstrated that the use of a gold carboxylate as a source of gold nanoparticles makes it possible to simply prepare a colloidal gold solution applicable for use in a wide variety of applications without fear of residual chloride ions and the like. 

1. A colloidal gold solution, comprising: water; gold nanoparticles with a particle size of 100 nm or less; and an anion represented by formula R—COO—  (a) wherein R represents a linear or branched alkyl group of 1 to 4 carbon atoms.
 2. The colloidal gold solution according to claim 1, wherein the anion is an acetate ion.
 3. The colloidal gold solution according to claim 1, further comprising a protective colloid.
 4. The colloidal gold solution according to any one of claim 1, wherein the protective colloid is at least one selected from the group consisting of polyvinylpyrrolidone, polyethylene glycol, polyacrylic acid, poly(sodium acrylate), polyvinyl alcohol, and carboxymethyl cellulose.
 5. The colloidal gold solution according to any one of claim 1, further comprising a reducing agent comprising an alcohol with a primary hydroxyl group and/or a secondary hydroxyl group.
 6. The colloidal gold solution according to any one of claim 1, wherein the gold nanoparticles have a concentration of 0.001 to 50% by weight.
 7. The colloidal gold solution according to any one of claim 1, which is substantially free of chloride ions.
 8. A method for producing a colloidal gold solution, comprising the steps of: (i) dispersing a gold carboxylate in water to form a dispersion; and (ii) allowing a reducing agent to react with the gold carboxylate in the dispersion obtained in the step (i) so that a colloidal gold solution is obtained by reduction.
 9. The method according to claim 8, wherein the reduction is performed using at least one selected from the group consisting of an alcohol with a primary hydroxyl group and/or a secondary hydroxyl group, polyvinylpyrrolidone, polyvinyl alcohol, polyethylene glycol, gelatin, starch, dextrin, carboxymethyl cellulose, methyl cellulose, and ethyl cellulose.
 10. The method according to claim 8, wherein the gold carboxylate is gold acetate.
 11. The method according to any one of claim 8, wherein the dispersion contains a protective colloid.
 12. The method according to any one of claim 8, wherein the protective colloid is at least one selected from the group consisting of polyvinylpyrrolidone, polyethylene glycol, polyacrylic acid, poly(sodium acrylate), polyvinyl alcohol, and carboxymethyl cellulose. 